Propionyl-L-carnitine-mediated improvement in contractile function of rat hearts oxidizing acetoacetate.

Prior evidence has suggested that propionyl-L-carnitine improves function in ischemic hearts by providing carnitine for dissipation of acyl-CoA derivatives and propionate for enrichment of the citric acid cycle. Because contractile failure in hearts oxidizing ketone bodies is due to sequestration of free coenzyme A, which can be reversed by the addition of anaplerotic substrates that enrich the citric acid cycle, experiments were performed to determine whether the addition of propionyl-L-carnitine (2 mM) can improve performance in working rat hearts utilizing acetoacetate (7.5 mM). Whereas the addition of propionyl-L-carnitine to acetoacetate resulted in a sustained improvement in the work output of the heart, the addition of propionate (2 mM) or L-carnitine (2 mM) alone to acetoacetate had negligible effects on contractile function. Propionyl-L-carnitine increased the uptake of acetoacetate by 130%, whereas beta-hydroxybutyrate release was minimal and unchanged compared with other groups. These observations show that rates of acetoacetate oxidation are increased commensurate with increased contractile function. Tissue metabolite data indicate that the utilization of propionyl-L-carnitine did not lead to accumulation of citric acid cycle intermediates in the span from citrate to 2-oxoglutarate but to an increase in the tissue content of malate. The results show that addition of propionyl-L-carnitine in hearts oxidizing acetoacetate results in improved mechanical performance that is comparable to the mechanical performance of hearts perfused with glucose as the only substrate. This improvement is most likely conferred by anaplerosis, as suggested by enhanced rates of acetoacetate utilization and citric acid flux.

Skeletal muscle glucose uptake during short-term contractile activity in vivo: effect of prior contractions.

The goal of the present study was to examine the time course of skeletal muscle glucose uptake and changes in intracellular metabolites occurring with the onset of in situ stimulation, and to assess the effect of a prior period of contractions on subsequent contraction-induced increases in glucose uptake. Hindlimb muscle in anesthetized rabbits was studied noninvasively using the positron-emitting glucose analog 18F-2-fluoro-deoxy-D-glucose (FDG). Fractional rates of FDG phosphorylation were measured on a minute-to-minute basis during rest, 3.5 minutes of priming exercise (PE), 15 or 30 minutes of PE recovery, and a subsequent 15-minute period of contractions. Muscles were electrically stimulated at 2 Hz, and force production was held constant during the contraction period(s). FDG uptake did not differ from control values either during PE or during 60 minutes of recovery from PE. In response to 15 minutes of contractions, muscle stimulated without PE demonstrated increased FDG uptake, but only after a delay of 5.0 +/- 0.7 minutes. Muscle with PE but rested 15 minutes had increased FDG uptake with a delay of 0.5 +/- 0.2 minutes, and muscle with PE but rested 30 minutes had increased FDG uptake after a delay of 8.0 +/- 0.9 minutes (P < .01 all groups). All groups reached similar levels of FDG uptake by the end of 15 minutes of contractions. Both groups with PE had control levels of adenosine triphosphate (ATP), phosphocreatine (PCr), and glucose-6-phosphate (G6P) after PE recovery, but glycogen level was lower than the control value (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Compartmentation of hexokinase in rat heart. A critical factor for tracer kinetic analysis of myocardial glucose metabolism.

Radiolabeled analogues of 2-deoxyglucose are widely used to trace glucose metabolism in cell cultures, whole organs, and intact animals, although kinetic differences in transport and phosphorylation between these compounds and glucose exist. The present studies were undertaken to determine the effects of insulin stimulation on the phosphorylation of 2-deoxyglucose compared to glucose in the intact, saline-perfused working rat heart. Rates of glucose utilization determined from tritiated glucose differed from rates estimated from the accumulation of [14C]2-deoxyglucose in a nonconstant manner when comparing rates in the absence or presence of physiologic levels of insulin (13 microU/ml). The fraction of monophosphorylated hexoses that was accounted for by [14C]2-deoxyglucose 6-phosphate was dramatically decreased in hearts perfused in the presence of insulin. Additionally, hexokinase activity associated with the mitochondrial fraction of tissue extracts was increased in hearts stimulated by insulin. While this redistribution of hexokinase to the mitochondria did not affect the apparent affinity constant for glucose, hexokinase bound to mitochondria exhibited an 8.5-fold decrease in the affinity for 2-deoxyglucose when compared with hexokinase present in the cytosolic fraction. The findings are consistent with an insulin-mediated preferential uptake and phosphorylation of glucose compared to deoxyglucose. The results also imply that the redistribution of hexokinase and the differential effect of insulin on its affinity for tracer and tracee are responsible for changes in the "lumped constant" (i.e., the correction factor used to equate 2-deoxyglucose to glucose uptake). These changes must be taken into account when regional myocardial glucose metabolism is assessed by the 2-deoxyglucose method.